Pressure gradient transducers



Dec. 11, 1962 M. M. ROSENFELD PRESSURE GRADIENT TRANSDUCERS Filed May 10, 1956 United States Patent Ofifice $368,328 Patented Dec. 11, 1962 3,368,323 PRESSURE GRADEENT TRANSDUCERS Murray M. Rosenr'eld, 1877 Gcean Ave., Brooklyn 30, NY. Filed May 10, 1955, Ser. No. 584,017 1 Claim. (El. 179-138) This invention relates to electroacoustical transducers of the pressure gradient type used as microphones.

Anti-noise Characteristics of Differential Microphones, by H. E. Ellithorn and A. M. Wiggins, in Proceedings of the I.R.E., February 1946, and in Elements of Acoustical Engineering, by H. F. Olson, 2nd edition, 1947, pages 237-276.

distance of an impinging sound wave. A second order pressure gradient microphone responds to the second derivative of pressure versus distance of an impinging sound wave. An nth order microphone responds to the nth derivative of pressure versus distance. A zero order microphone is a pressure microphone and responds to the pressure variations of an impinging sound wave, without regard to its direction or gradient.

Generally, a first order pressure gradient microphone may consist of two pressure microphones spaced some distance apart or a single element responsive to the sound wave. A second order It is an object of this invention to provide a second order pressure gradient responsive microphone which has the following advantages: (a) uses one diaphragm; (b) is economical and simple to construct; (c) is extremely simple to balance; (d) has higher discrimination than first order and present types of 2nd order microphones, particularly at higher frequencies; (e) is small in size; (f) if unbalanced, behaves as a first order microphone.

The novel features of my invention, as well as additional objects and advantages thereof, will be understood better from the following description of several embodiments thereof, when read in connection with the accompanying drawings in which:

FIGURES l, 2 and 3 are plan, front and side elevation views respectively of one form of second order pressure gradient transducer.

FIGURE 4 is a plan view of the transducer of FIG- URES 1, 2 and 3 showing the relation of pressures on the respective diaphragnis.

FIGURE 5 is a plan view of the directional characteristics of the transducer of FIGURES 1, 2, and 3.

FIGURES 6, 7 and 8 are plan, front and side elevation views, respectively, of another form of second order pressure gradient transducer.

FIGURE 9 is a plan view of the directional characteristics of the transducer of FIGURES 6, 7 and 8.

FlGURES 10, 11 and 12 are plan, from and side elevation views respectively, of still another form of second order press-gradient transducer which uses a single diaphragm plate.

constructed by using two matched diaphragm plates. The two equal diaphragm plates 5 and 6 are arranged as shown in FIGURES l, 2 and 3, with a connecting rod 8 joining an edge of each and perpendicular to the planes of the two diaphragm plates. Through the center of the connecting rod 8 and perpendicular to it, another connecting rod 7 18 attached which serves as an axis of rotation.

of rotation into electrical output.

FIGURE 4 is another top view of the microphone of FIGURES 1, 2, and 3 and shows the relation of pressures on the diaphragm plates when a sound wave impinges normal to the diaphragms. The torque tending to produce rotation of the microphone assembly about the axis of rotation will be:

Torque about aXis=L((P P )-(P -P where P is the pressure at the front of the first diaphragm plate P is the pressure at the rear of the first diaphragm plate P is the pressure at the front of the second diaphragm plate P is the pressure at the rear of the second diaphragm plate L is the effective distance between the axis of rotation and the resultant sound pressure f. the sound wave is traveling in the direction shown by the arrow in FIGURE 4, P P P P and the direction of rotation would be counter clockwise. Sounds from nearby sources would produce greater torques than those from distant sources because the pressure differentials on the diaphragms would be greater.

The microphone in FIGURES 1, 2 and 3 will have a response dependent upon the difference between the pressure differentials at each of the two diaphragm plates, in other words, the second order of pressure gradient. It will exhibit the directional characteristics shown in FIGURE 5.

Another form of the microphone in FEGURES 1, 2 and 3 is shown in FIGURES 6, 7 and 8. In FIGURES 6, 7 and 8 the diaphragm plates 10 and Ill are in the same plane, the connecting rod is 13, the axis of rotation is 12, and the electrical generating means is 7.4. The micro- The two second order pressure gradient microphones in FIGURES l, 2, 3, and 6, 7, 8 require careful matching of diaphragm plates and balancing about the axes of rotation. A simpler second order pressure gradient microphone, using only a single diaphragm, can be developed using the same principle and is shown in FIGURES 10, 11 and 12. It uses a single diaphragm 15 with the axis of rotation 16 through its plane symmetrically dividing it into two equal parts. A connecting rod 17 passing through this axis of rotation, connects the diaphragm to electrical generating means 18 which converts angular rotation of the connecting rod into electrical output. When a sound wave impinges upon the microphone of FIGURES 10, 11 and 12 there will be a pressure differential on each half of the diaphragm which will produce torques about the axis of rotation, tending to oppose one another. There will be certain directions of impinging sound waves which will produce the greatest electrical output from the microphone in FIGURES 10, 11 and 12. Its directional characteristics are shown in FIGURE 13. The second order pressure gradient microphone of FIG- URES 10, 11 and 12 is simple to construct and would show extremely good discrimination against distant sounds at both low and high frequencies, thereby being superior to a first order type. If unbalanced it would behave as a first order type.

A second order pressure gradient microphone can be constructed which uses a twister type bimorphic or multielement piezoelectric crystal for a diaphragm plate 19 as shown in FIGURES 14 and 15. Here the diaphragm plate is not separated from the electrical generating means as in the microphone of FIGURES 10, 11 and 12 but both are one and the same. In the microphone of FIG- URES 14 and 15, the net torque acting upon the diaphragrn plate piezoelectric crystal is dependent upon the second order of pressure gradient. This torque produces corresponding electrical output from the twister type piezoelectric element. The directional characteristics would be similar to that shown in FIGURE 13.

Although this invention has been disclosed with reference to a number of specific embodiments, it will be evident to those skilled in the art that additional modifications and applications of the principles of the invention may be made without departing from the spirit or the scope thereof.

1 claim as my invention:

In a transducer, a second order pressure gradient microphone consisting of a substantially symmetric diaphragm plate, electrical generating means peculiarly responsive to angular vibratory motion, means for connecting said diaphragm plate to said electrical generating means collinearly with an aXis of symmetry of said diaphragm plate, said diaphragm plate suspended in air by said connecting means allowing free access of sound waves to all parts of said diaphragm plate.

References Cited in the file of this patent UNITED STATES PATENTS OTHER REFERENCES Elements of Acoustical Engineering, Olson (D. Van Nostrand 00.), published 1947, 

